Conventional transmission electron microscopy (TEM) has become an indispensable tool for comprehensive nanoscale materials characterization. While TEM spatial and energy resolutions have reached half-angstrom and few-meV levels, respectively, state-of-the-art detectors are able to resolve dynamics occurring only on the order of milliseconds. Such temporal resolutions are insufficient for studying a wealth of charge-carrier, structural, and magnetic behaviors. To overcome this, stroboscopic pump/probe approaches have been developed by interfacing a conventional TEM with short-pulsed lasers. In this way, temporal resolutions can be improved by 10 orders of magnitude to sub-picosecond timescales. In this talk, I will describe our work on the development and the application of this approach – ultrafast electron microscopy (UEM). I will begin by providing an overview of the UEM methodology and technology specific to our lab. Following this, I will describe a selection of our results on resolving the influence of nanoscale structural discontinuities (e.g., interfaces and crystal terraces) on coherent, elastic strain-wave dynamics in layered materials. Among other behaviors, we find that wave-train emergence occurs at extended discontinuities, with propagation directions oriented normal to the interface, independent of in-plane crystallographic direction. Properties of the wave trains (GHz frequencies, speed-of-sound velocities, and single in-plane wave directions) suggest the generation of a single acoustic-phonon mode following photoexcitation, with observable interference effects occurring at vacuum/crystal interfaces. I will conclude by sharing our most recent results on photoinduced coherent strain waves in undoped Ge, wherein the precise behaviors (e.g., hypersonic phase velocities, time-varying velocity dispersion, delayed onset relative to femtosecond photoexcitation, etc.) bear striking similarities to coherent, photogenerated plasma waves in Ge and GaAs. The objective of the talk is to give a sense of the capabilities of the UEM lab at the University of Minnesota and to provide an overview of the ongoing work in my group, which is currently focused on understanding high-velocity, coherent strain waves, and the corresponding roles of defects, in nanoscale semiconducting and metallic materials.
Seminars are open to alumni, friends of the Department, and the general public.